1. Field of the Invention
Gradient refractive index optical devices with particular reference to improvements in method of making radially graded refractive index rods, fibers and devices therefrom.
2. Discussion of the Prior Art
A radially graded refractive index fiber or rod has a light focusing power similar to a positive lens or a series of lenses, e.g. a rod as small as 1 mm in diameter can relay a distinct image of an object placed at or near the entrance to the rod.
One method by which gradient index rods or fibers have been prepared includes immersion (steeping) of a rod of glass containing thallium oxide (Tl.sub.2 O) in a molten bath of potassium nitrate (KNO.sub.3) at a temperature higher than the annealing temperature of the glass but lower than its deformation temperature. A typical cycle for this ion exchange process, by which potassium ions replace thallium ions by slow interdiffusion, is 432 hours at 460.degree. C. for a 1 mm diameter rod.
Other unduly tedious and costly methods of treating glass rods or fibers with pastes, powders, liquids and/or gasses for effecting refractive index gradation are exemplified in U.S. Pat. No. 3,843,228 but wherein, as before mentioned, treating temperatures must be maintained below the rod or fiber deformation temperature, i.e. deformation being unacceptable.
With a view to minimizing product costs, simplifying processing techniques and providing for efficient mass production of gradient refractive index rods, fibers and products thereof, a principal object of this invention is to make possible the use of ion diffusion temperatures which are higher than deformation temperatures of articles being treated without permitting deformation of the articles.
In addition to shortening ion diffusion times by using higher than deformation temperatures, another object is to provide for the mass simultaneous ion exchange treatment of large numbers of glass rods or fibers, i.e. bundles thereof adapted to be subsequently readily separated from one another.
Still another object is to accomplish the exchange of ions needed for refractive index gradation in glass rods or fibers with a solid exchange agent having such compatibility with the rod or fiber as to permit simultaneous drawing or shaping of the rod or fiber with the exchange agent prior to effecting ion exchange treatment and wherewith the need to deal with heretofore messy and difficult to handle ion exchange agents and processing techniques is avoided.
Other objects and advantages of the invention will become apparent from the following description.